You can think of tristate buffers as a way of turning a signal on and off. When the enable input at the top of the buffer is '1', the tristate buffer acts like a normal buffer. But when the enable input is '0', the buffer "turns off" by giving a very high impedance output. This effectively "disconnects" the buffer from the circuit. So if you need to turn off a signal, ground the enable input of the tristate buffer.